package info.opencards.pptintegration.conversion;

import java.io.*;
import java.nio.charset.Charset;
import java.util.Collection;


/**
 * <p>Utility class to guess the encoding of a given byte array. The guess is unfortunately not 100% sure. Especially
 * for 8-bit charsets. It's not possible to know which 8-bit charset is used. Except through statistical analysis. We
 * will then infer that the charset encountered is the same as the default standard charset.</p> <p/> <p>On the other
 * hand, unicode files encoded in UTF-16 (low or big endian) or UTF-8 files with a Byte Order Marker are easy to find.
 * For UTF-8 files with no BOM, if the buffer is wide enough, it's easy to guess.</p> <p/> <p>Tested against a
 * complicated UTF-8 file, Sun's implementation does not render bad UTF-8 constructs as expected by the specification.
 * But with a buffer wide enough, the method guessEncoding() did behave correctly and recognized the UTF-8 charset.</p>
 * <p/> <p>A byte buffer of 4KB or 8KB is sufficient to be able to guess the encoding.</p> <p/> <p>Usage:</p>
 * <pre>
 * // guess the encoding
 * Charset guessedCharset = com.glaforge.i18n.io.CharsetToolkit.guessEncoding(file, 4096);
 * <p/>
 * // create a reader with the charset we've just discovered
 * FileInputStream fis = new FileInputStream(file);
 * InputStreamReader isr = new InputStreamReader(fis, guessedCharset);
 * BufferedReader br = new BufferedReader(isr);
 * <p/>
 * // read the file content
 * String line;
 * while ((line = br.readLine())!= null)
 * {
 *     System.out.println(line);
 * }
 * </pre>
 * <p/> <p>Date: 18 juil. 2002</p>
 *
 * @author Guillaume LAFORGE
 */
public class CharsetToolkit {

    private final byte[] buffer;
    private Charset defaultCharset;
    private boolean enforce8Bit = false;


    /**
     * Constructor of the <code>com.glaforge.i18n.io.CharsetToolkit</code> utility class.
     *
     * @param buffer the byte buffer of which we want to know the encoding.
     */
    private CharsetToolkit(byte[] buffer) {
        this.buffer = buffer;
        this.defaultCharset = getDefaultSystemCharset();
    }


    /**
     * Constructor of the <code>com.glaforge.i18n.io.CharsetToolkit</code> utility class.
     *
     * @param buffer         the byte buffer of which we want to know the encoding.
     * @param defaultCharset the default Charset to use in case an 8-bit charset is recognized.
     */
    public CharsetToolkit(byte[] buffer, Charset defaultCharset) {
        this.buffer = buffer;
        setDefaultCharset(defaultCharset);
    }


    /**
     * Defines the default <code>Charset</code> used in case the buffer represents an 8-bit <code>Charset</code>.
     *
     * @param defaultCharset the default <code>Charset</code> to be returned by <code>guessEncoding()</code> if an 8-bit
     *                       <code>Charset</code> is encountered.
     */
    void setDefaultCharset(Charset defaultCharset) {
        if (defaultCharset != null)
            this.defaultCharset = defaultCharset;
        else
            this.defaultCharset = getDefaultSystemCharset();
    }


    /**
     * If US-ASCII is recognized, enforce to return the default encoding, rather than US-ASCII. It might be a file
     * without any special character in the range 128-255, but that may be or become a file encoded with the default
     * <code>charset</code> rather than US-ASCII.
     *
     * @param enforce a boolean specifying the use or not of US-ASCII.
     */
    public void setEnforce8Bit(boolean enforce) {
        this.enforce8Bit = enforce;
    }


    /**
     * Gets the enforce8Bit flag, in case we do not want to ever get a US-ASCII encoding.
     *
     * @return a boolean representing the flag of use of US-ASCII.
     */
    public boolean getEnforce8Bit() {
        return this.enforce8Bit;
    }


    /**
     * Retrieves the default Charset
     *
     * @return
     */
    public Charset getDefaultCharset() {
        return defaultCharset;
    }


    /**
     * <p>Guess the encoding of the provided buffer.</p> If Byte Order Markers are encountered at the beginning of the
     * buffer, we immidiately return the charset implied by this BOM. Otherwise, the file would not be a human readable
     * text file.</p> <p/> <p>If there is no BOM, this method tries to discern whether the file is UTF-8 or not. If it
     * is not UTF-8, we assume the encoding is the default system encoding (of course, it might be any 8-bit charset,
     * but usually, an 8-bit charset is the default one).</p> <p/> <p>It is possible to discern UTF-8 thanks to the
     * pattern of characters with a multi-byte sequence.</p>
     * <pre>
     * UCS-4 range (hex.)        UTF-8 octet sequence (binary)
     * 0000 0000-0000 007F       0xxxxxxx
     * 0000 0080-0000 07FF       110xxxxx 10xxxxxx
     * 0000 0800-0000 FFFF       1110xxxx 10xxxxxx 10xxxxxx
     * 0001 0000-001F FFFF       11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
     * 0020 0000-03FF FFFF       111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     * 0400 0000-7FFF FFFF       1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     * </pre>
     * <p>With UTF-8, 0xFE and 0xFF never appear.</p>
     *
     * @return the Charset recognized.
     */
    public Charset guessEncoding() {
        // if the file has a Byte Order Marker, we can assume the file is in UTF-xx
        // otherwise, the file would not be human readable
        if (hasUTF8Bom(buffer)) return Charset.forName("UTF-8");
        if (hasUTF16LEBom(buffer)) return Charset.forName("UTF-16LE");
        if (hasUTF16BEBom(buffer)) return Charset.forName("UTF-16BE");

        // if a byte has its most significant bit set, the file is in UTF-8 or in the default encoding
        // otherwise, the file is in US-ASCII
        boolean highOrderBit = false;

        // if the file is in UTF-8, high order bytes must have a certain value, in order to be valid
        // if it's not the case, we can assume the encoding is the default encoding of the system
        boolean validU8Char = true;

        // note the buffer is not read up to the end, but up to length - 6

        int length = buffer.length;
        int i = 0;
        while (i < length - 6) {
            byte b0 = buffer[i];
            byte b1 = buffer[i + 1];
            byte b2 = buffer[i + 2];
            byte b3 = buffer[i + 3];
            byte b4 = buffer[i + 4];
            byte b5 = buffer[i + 5];
            if (b0 < 0) {
                // a high order bit was encountered, thus the encoding is not US-ASCII
                // it may be either an 8-bit encoding or UTF-8
                highOrderBit = true;
                // a two-bytes sequence was encoutered
                if (isTwoBytesSequence(b0)) {
                    // there must be one continuation byte of the form 10xxxxxx,
                    // otherwise the following characteris is not a valid UTF-8 construct
                    if (!isContinuationChar(b1))
                        validU8Char = false;
                    else
                        i++;
                }
                // a three-bytes sequence was encoutered
                else if (isThreeBytesSequence(b0)) {
                    // there must be two continuation bytes of the form 10xxxxxx,
                    // otherwise the following characteris is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2)))
                        validU8Char = false;
                    else
                        i += 2;
                }
                // a four-bytes sequence was encoutered
                else if (isFourBytesSequence(b0)) {
                    // there must be three continuation bytes of the form 10xxxxxx,
                    // otherwise the following characteris is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2) && isContinuationChar(b3)))
                        validU8Char = false;
                    else
                        i += 3;
                }
                // a five-bytes sequence was encoutered
                else if (isFiveBytesSequence(b0)) {
                    // there must be four continuation bytes of the form 10xxxxxx,
                    // otherwise the following characteris is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2) && isContinuationChar(b3) && isContinuationChar(b4)))
                        validU8Char = false;
                    else
                        i += 4;
                }
                // a six-bytes sequence was encoutered
                else if (isSixBytesSequence(b0)) {
                    // there must be five continuation bytes of the form 10xxxxxx,
                    // otherwise the following characteris is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2) && isContinuationChar(b3) && isContinuationChar(b4) && isContinuationChar(b5)))
                        validU8Char = false;
                    else
                        i += 5;
                } else
                    validU8Char = false;
            }
            if (!validU8Char) break;
            i++;
        }
        // if no byte with an high order bit set, the encoding is US-ASCII
        // (it might have been UTF-7, but this encoding is usually internally used only by mail systems)
        if (!highOrderBit) {
            // returns the default charset rather than US-ASCII if the enforce8Bit flag is set.
            if (this.enforce8Bit)
                return this.defaultCharset;
            else
                return Charset.forName("US-ASCII");
        }
        // if no invalid UTF-8 were encountered, we can assume the encoding is UTF-8,
        // otherwise the file would not be human readable
        if (validU8Char) return Charset.forName("UTF-8");
        // finally, if it's not UTF-8 nor US-ASCII, let's assume the encoding is the default encoding
        return this.defaultCharset;
    }


    private static Charset guessEncoding(File f, int bufferLength) throws IOException {
        FileInputStream fis = new FileInputStream(f);
        byte[] buffer = new byte[bufferLength];
        fis.read(buffer);
        fis.close();

        CharsetToolkit toolkit = new CharsetToolkit(buffer);
        toolkit.setDefaultCharset(getDefaultSystemCharset());

        return toolkit.guessEncoding();
    }


    public static Charset guessEncoding(File f, int bufferLength, Charset defaultCharset) throws IOException {
        FileInputStream fis = new FileInputStream(f);
        byte[] buffer = new byte[bufferLength];
        fis.read(buffer);
        fis.close();
        CharsetToolkit toolkit = new CharsetToolkit(buffer);
        toolkit.setDefaultCharset(defaultCharset);
        return toolkit.guessEncoding();
    }


    /**
     * If the byte has the form 10xxxxx, then it's a continuation byte of a multiple byte character;
     *
     * @param b a byte.
     * @return true if it's a continuation char.
     */
    private static boolean isContinuationChar(byte b) {
        return -128 <= b && b <= -65;
    }


    /**
     * If the byte has the form 110xxxx, then it's the first byte of a two-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a two-bytes sequence.
     */
    private static boolean isTwoBytesSequence(byte b) {
        return -64 <= b && b <= -33;
    }


    /**
     * If the byte has the form 1110xxx, then it's the first byte of a three-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a three-bytes sequence.
     */
    private static boolean isThreeBytesSequence(byte b) {
        return -32 <= b && b <= -17;
    }


    /**
     * If the byte has the form 11110xx, then it's the first byte of a four-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a four-bytes sequence.
     */
    private static boolean isFourBytesSequence(byte b) {
        return -16 <= b && b <= -9;
    }


    /**
     * If the byte has the form 11110xx, then it's the first byte of a five-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a five-bytes sequence.
     */
    private static boolean isFiveBytesSequence(byte b) {
        return -8 <= b && b <= -5;
    }


    /**
     * If the byte has the form 1110xxx, then it's the first byte of a six-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a six-bytes sequence.
     */
    private static boolean isSixBytesSequence(byte b) {
        return -4 <= b && b <= -3;
    }


    /**
     * Retrieve the default charset of the system.
     *
     * @return the default <code>Charset</code>.
     */
    private static Charset getDefaultSystemCharset() {
        return Charset.forName(System.getProperty("file.encoding"));
    }


    /**
     * Has a Byte Order Marker for UTF-8 (Used by Microsoft's Notepad and other editors).
     *
     * @param bom a buffer.
     * @return true if the buffer has a BOM for UTF8.
     */
    private static boolean hasUTF8Bom(byte[] bom) {
        return (bom[0] == -17 && bom[1] == -69 && bom[2] == -65);
    }


    /**
     * Has a Byte Order Marker for UTF-16 Low Endian (ucs-2le, ucs-4le, and ucs-16le).
     *
     * @param bom a buffer.
     * @return true if the buffer has a BOM for UTF-16 Low Endian.
     */
    private static boolean hasUTF16LEBom(byte[] bom) {
        return (bom[0] == -1 && bom[1] == -2);
    }


    /**
     * Has a Byte Order Marker for UTF-16 Big Endian (utf-16 and ucs-2).
     *
     * @param bom a buffer.
     * @return true if the buffer has a BOM for UTF-16 Big Endian.
     */
    private static boolean hasUTF16BEBom(byte[] bom) {
        return (bom[0] == -2 && bom[1] == -1);
    }


    /**
     * Retrieves all the available <code>Charset</code>s on the platform, among which the default <code>charset</code>.
     *
     * @return an array of <code>Charset</code>s.
     */
    public static Charset[] getAvailableCharsets() {
        Collection collection = Charset.availableCharsets().values();
        return (Charset[]) collection.toArray(new Charset[collection.size()]);
    }


    public static void main(String[] args) throws IOException {
//		File file = new File("utf-8.txt");
        File file = new File("windows-1252.txt");

        Charset guessedCharset = CharsetToolkit.guessEncoding(file, 4096);
        System.err.println("Charset found: " + guessedCharset.displayName());

        FileInputStream fis = new FileInputStream(file);
        InputStreamReader isr = new InputStreamReader(fis, guessedCharset);
        BufferedReader br = new BufferedReader(isr);

        String line;
        while ((line = br.readLine()) != null) {
            System.out.println(line);
        }

    }
}
